Evidence of Nuclear Rainbow in the 16O + 27Al elastic scattering @ 280 MeV

Abstract

In the present work the 16O + 27Al elastic scattering at 280 MeV incident energy is investigated. This study is inserted in the research line aiming at the investigation of the nuclear rainbow in the scattering process involving heavy nuclei. Nuclear rainbow is a peculiar phenomenon which reveals the nuclear interaction at small distances, where the projectile and target density overlaps can reach values up to twice the nuclear matter saturation value. The rainbow features were thought to be very unlikely for systems heavier than 16O + 16O because of the strong absorption, always present in heavy-ion collisions, which tends to hide the refractive component of the scattering process, responsible for the rainbow phenomenon. The experiment was performed at the Catania LNS-INFN laboratory using a 280 MeV energy 16O Cyclotron beam impinging on a 109 ug/cm2 thick 27Al target. The ejectiles were momentum analysed by the MAGNEX large acceptance magnetic spectrometer and detected by its Focal Plane Detector. In order to obtain wide elastic and inelastic angular distributions, five different angular settings were chosen, which covered a whole angular range from about 5° to about 40° in the laboratory reference frame. 27Al excitation energy spectra up to 85 MeV were extracted, showing the population of the ground state and low-lying excited states of the residual nucleus. In addition, the spectra measured at forward angles showed the population of large structures in a narrow angular range at relatively-high excitation energy, corresponding to the excitation of collective modes of the target nucleus, i.e. Monopole and Quadrupole Giant Resonances. Then, elastic and inelastic cross section angular distributions were constructed for each angular setting. A model independent data analysis was performed. The total elastic 16O + 27Al angular distribution was fitted using exponential functions in three different angular ranges. This procedure made it possible to better appreciate the systematic deviation of the data with respect to the rapid exponential decrease expected for a strong absorptive system. The rise of the cross section observed at the largest scattering angles is the evidence of a well developed nuclear rainbow structure for the investigated system. The elastic and inelastic angular distributions were then compared with a new generation of parameter-free calculations based on the Coupled Channel (CC) formalism, using the Sao Paulo potential (SPP). The calculations were performed with the computer code FRESCO. A good agreement with the data was obtained using a reduced real component of the nucleus-nucleus potential and adopting the weak coupling model. In such a model, the 27Al ground state and the first low lying excited states (1/2+ , 3/2+ , 5/2+ , 7/2+ , 9/2+) were described as a 1d5/2 proton hole coupled with the 28Si 0+ ground state and 2+ (1.779 MeV) excited state, respectively. The effect of the inclusion in the calculation of other 28Si excited states was also investigated in a preliminary way.